Rethinking aromaticity in H-bonded systems. Caveats for transition structures involving hydrogen transfer and...

Monoaza- and diaza-derivatives of malondialdehydes, in short aminoacroleins and vinamidines, are prototypical examples of open-chain structures prone to π-electron delocalization, for which intramolecular hydrogen bonding enhances (or diminishes) their pseudoaromaticity depending on the substitution pattern. This interplay is illustrated herein by DFT-based calculations of aromaticity indices in the gas phase and polar solvents. Elucidation of transition structures involved in tautomeric conversions helps to solve how the intramolecular hydrogen transfer occurs. While TSs exhibit a high degree of aromaticity, the dichotomy between forward and backward pathways points to a complex trajectory. Addition of thermal corrections to the electronic energy decreases both the enthalpy and free energy leading to negative ΔH(‡) and ΔG(‡) values. This variational effect accounts for the otherwise elusive distinction between transition structures and saddle points (usually overlooked for high electronic barriers). Also, this rationale fits well within the framework of Marcus' theory.